Fix attribute error when saving mod that was in deleted modpack

[HebaruSan]

Problem

1. Create a mod pack
2. Add a mod
3. Delete the mod pack
4. Try to edit the mod from step 2
5. Save your changes
6. 
   

Cause

In #368 we added some code that assumes that ModListItem.mod_list_id won't be NULL.

SpaceDock/KerbalStuff/search.py

Line 28 in 453f8d5

score += 15 * len({itm.mod_list for itm in mod.mod_list_items if itm.mod_list.user_id != mod.user_id})

However, when you delete a mod list, its ModListItems are not deleted but simply have their mod_list_id set to NULL.

spacedock=# select * from modlistitem;
 id | mod_id | mod_list_id | sort_index 
----+--------+-------------+------------
  1 |      1 |             |          0
  2 |      2 |             |          1
  3 |      3 |             |          2
  4 |      3 |             |          0
  6 |      2 |             |          1
  5 |      1 |           4 |          0
  7 |      2 |           4 |          1
  8 |      3 |           6 |          0
  9 |      5 |           4 |          2
 10 |      6 |           4 |          3
(10 rows)

Each such row represents a case where Mod.mod_list_items[X].mod_list is None for some mod.

Changes

• Now all ModListItems from deleted mod lists are deleted
• Now ModListItem.mod_id is non-nullable and has cascade deletion enabled
• Now ModListItem.mod_list_id is non-nullable and has cascade deletion enabled
• Now ModListItem.sort_index is non-nullable

After this, the code from #368 will be correct that ModListItem.mod_list_id won't be NULL.

[DasSkelett]

Thanks, works perfectly! non-nullable-ify all the things!